Developing Fragility Curves for Seismic Vulnerability Assessment of Tubular Steel Power Transmission Tower Based on Incremental Dynamic Analysis

Transcription

1 Developng Fraglty Curves for Sesmc Vulnerablty Assessment of Tubular Steel Power Transmsson Tower Based on Incremental Dynamc Analyss M. Sadegh Department of Structural Engneerng, Scence and Research Branch, Islamc Azad Unversty, Tehran, Iran. M. Hossen Structural Engneerng Research Center, Internatonal Insttute of Earthquake Engneerng and Sesmology (IIEES), Tehran, Iran. N. Pakdel Lahj Department of Structural Engneerng, Scence and Research Branch, Islamc Azad Unversty, Tehran, Iran. SUMMARY: Accordng to the experence on vulnerablty of power networks n past earthquakes, t has been learned that the contnuous operaton of power network, especally power transmsson towers, can help better emergency response and relef operatons. Regardng to durablty and ease of manufacturng and nstallaton, many utltes n recent years, prefer to use the tubular steel towers rather than over lattce steel for new power lnes and tower replacements. Ths study focuses on developng fraglty curves of tubular steel power transmsson towers. The man goal of ths research s to assess the sesmc performance relablty of tubular steel power transmsson towers subjected to earthquake loadngs by probablstc approach. Fraglty functons,whch provde the probablty of exceedng a prescrbed level of damage for a wde range of ground moton ntenstes, are the major requrements for sesmc loss estmaton and are wdely used n the sesmc rsk management and related studes. In fraglty curves, structural demand and ground moton ntensty measure are two key parameters for determnng exceedance probablty of predefned performance level. In ths research, dsplacement and spectral acceleraton have been selected as structural demand and ground moton ntensty measure, respectvely. Due to lack of knowledge about defnng structural lmt states for tubular steel towers, nonlnear statc pushover analyss has been conducted and then by corporaton analyss results wth engneerng judgment, performance levels have been chosen. Seven far-source earthquakes, n the g PGA range, have been selected for conductng nonlnear dynamc analyss. It s notable that selected records are compatble wth ste specfcatons. By performng ncremental dynamc analyss (IDA) for each ground moton record, fraglty curves have been developed. Fnally, the results have been dscussed by nterpretng the fraglty curves. Keywords: Fraglty Curves, Rsk Management, Power Transmsson Tower, Incremental Dynamcs Analyss, Nonlnear statc pushover analyss 1. INTRODUCTIONS The electrc lnes that generate the most publc nterest are hgh-voltage transmsson lnes. These are the largest and most vsble electrc lnes. Most large ctes requre several transmsson lnes for relable electrc servces. The amount of electrc power n each lne, at any gven moment, depends on generaton producton and dspatch, Customer use, the status of other transmsson lnes and ther assocated equpments and even the weather. Earthquake causes extensve drect and ndrect losses n whch the damages to lfelnes play an mportant role. Power network and ts senstve components are key elements among all other groups of lfelnes such as telecommuncaton and transportaton networks, ppelnes, etc. It has been learned from the past earthquakes that the exstence of power network, especally transmsson towers, can facltate the rescue and relef operatons and lead to more savng lves [Sadegh et all 2010]. Accordng to the experence on vulnerablty of power networks n past earthquakes, t has been learned that the contnuous operaton of power network, especally power transmsson towers, can help better emergency response and relef operatons. Regardng to durablty and ease of manufacturng and nstallaton, many utltes n recent years,

2 prefer to use the tubular steel towers rather than over lattce steel for new power lnes and tower replacements. Ths study focuses on developng fraglty curves of tubular steel power transmsson towers based on probablstc approach. Several researchers have studed the effects of earthquake on transmsson towers. Ghobarah, Azz, and El-Attar (1996), L, Sh, and Ja(2003), L, Sh, Wang, and Ja(2005),L Tan, Hongnan L, and Gouhuan Lu (2010) have studed the effect of earthquake loadng on transmsson towers. Yn, L, Lu, and Zha (2005), (2005) have studes sesmc damages on power transmsson towers; but none of these mentoned researches have any rsk management horzon as a part of urban lfelnes. Ths paper tres to use a probablstc approach to use on past earthquakes rsk management by developng fraglty functons. 1. POWER TRANSMISSION TOWER Transmsson lnes are larger than common dstrbuton lnes that exst along rural roads and cty streets. Transmsson lne poles or structures are between 18 to 42 meters tall. Common 63Kv Dstrbuton lne structures are approxmately 20 meters tall. The transmsson tower s an mportant accessory and performance of the transmsson lne that depends very much on the desgn of the transmsson tower. The electrc transmsson towers or pylons can be classfed several ways. The most obvous and vsble tower types are: - Tubular steel tower - Wast-type tower - Double-crcut tower - Guyed-V tower - Guyed cross-rope suspenson tower - Crossngs tower 1.1. Tubular steel tower In many cases due to publc resentment, the use of lattce structures has been restrcted. Some power companes for hgh and extra hgh-tenson transmsson lnes have used steel tubular pole structures qute successfully. The nstallaton of these structures s costly but requres less tme. The tubular structure can be a sngle tubular form or H-form. It can be desgned for carryng two or more crcuts. More transmsson companes have started usng of ths type of tower especally n populated areas. Fgure 1. Tubular power transmsson tower The use of Tubular steel tower shape, heght and sturdness (mechancal strength) depend on the stresses to whch they are exposed. In some areas where sngle-pole structures are preferred, weak or

3 wet sols may requre concrete foundatons for support. A tubular power transmsson tower conssts of multple parts, most of whch owned by dfferent companes. Regularly ths tower conssts of the followng parts: Body, Arms, and Insulators. 2. AN OVERVIEW ON DEFINITION OF FRAGILITY FUNCTION AND ESTIMATION METHOD Due to practcal reasons, contnuous damage n structures s dvded nto several dscrete damage states [Porter 2000]. Fraglty functon estmates the condtonal exceedng probablty of damage from a damage state at gven ground moton ntensty: F(m) = P(D > d IM = m) (2.1) Where, F (m) s the probablty of exceedng damage D from damage state d at gven ground moton IM=m. Ground moton ntensty parameters denote the magntude of ground moton, whch s measured by Peak Ground Acceleraton (PGA), Peak Ground Velocty (PGV) or Spectral Dsplacement (SD). Damage state s defned by non-damage state (=0) to the n th damage state (=n) by qualtatve and analytcal defntons [HAZUS 1999].Snce damage n structures, n ths study, s measured by Damage Index (DI), Equaton1 s changed to: F(m) = P(DI > d IM = m) (2.2) t Where, d t s the damage ndex at the threshold of damage states. By Havng the Probablty Densty functon (PDF) of DI or ts cumulatve dstrbuton functon (CDF) at every m (fm (d) and (t) F m (dt)), Eqn 2.2 s evaluated from probablstc theorem: F(m) P(DI d IM m) 1 F (d ) 1 f (d)d(d) = > = = = (2.3) t m t m In ths paper, PDF of DI s evaluated by mult-strpe analyss [Aslan and Mranda 2004 and Jalayer 2003]. Here, transmsson tower s analyzed subjected to several ground moton records that are scaled to specfc IM (here PGA) level and dstrbuton of structural response n the partcular IM (for structural case) s estmated from the results of the nonlnear analyss set. Based on these assumptons, procedure of fraglty curve development for real structure(s) s summarzed n three major steps shown n flowchart of methodology gven n Fgure 2[Nasserasad et all 2009]. d t Probablty of Falure PGA Fgure 2. Developng Fraglty curve procedure [ Nasserasad et all 2009].

4 3. STATEMENT OF THE PROBLEM In ths secton, case study tubular tower 19.5m heght, whch s common for 63Kv transmsson lnes, has been dscussed. It s apparent that these transmsson towers play vtal role to have contnues electrcty after an earthquake. The analyses are performed by advanced fnte element software; for nvestgatng structural performance, Maxmum Base shear s derved. Ths s an mportant structural parameter, whch can descrbe the sustanablty of the tower. Fnally, wth a set of data, fraglty curves are developed and dscussed Structural modellng Refer to the techncal documents, materal of tower members s ST-37 and ST-52 steel grade wth yeldng stress F y =2400 Kg/cm² and F y =3600 Kg/cm² respectvely. Modulus of elastcty s E= Kg/cm² and passon rato equals to ν=0.3. Accordng to structural detals, fxed support s assgned n base and arms modelng. The desgn of transmsson tower and lne s complex and needs to consder loadng under dfferent condtons. Specfcaton of ths tower s llustrated n Table1. Table 1. Tubular Tower (19.5m) Detalng Parts Heght(mm) Intal Wdth(mm) End Wdth(mm) Steel Grade 1st ST-52 2nd ST-52 3rd ST-37 4th ST-37 5m 0.445m 2.20m 1.6m 1.6m 2.20m 0.445m 4.6m 0.645m 19.5m 4.4m 0.845m 5.5m 1.07m (a) (b) (c) Fgure 3. Software modelng (a), Structural detal (b) and real monopole tower(c). Pole heght and load capacty lmtatons, control allowable span length ether on the bass of ground clearance or ablty to support heavy wnd and ce loads. Hgh Wnd, Heavy Ice and Normal Wnd-Ice are three loadng cases, whch are used for analyss. Regardng to Iranan gudelne No.402 for desgnng the power transmsson lnes, heavy ce s selected as crtcal load case. Two condton are consdered for ths case; balance loadng (forces are appled two sde of the tower) and unbalance

5 loadng (forces are appled one sde of the tower). Therefore, Kg vertcal and Kg Horzontal force are appled on each tower s arms (see Table 2). Theses loadng are calculated for 63Kv Dstrbuton lne wth length of 19 Km and desgn span equals to 250 meter. Maxmum transmsson power s 20 Mega Watts. Insulator type s U120B and maxmum ce thckness s 20mm. Table 2. Heavy Ice Loadng Detals Sesmc nput Load Drecton Balance Force(Kg) Unbalance Force(Kg) Horzontal Vertcal Seven earthquake records are selected to be employed as the tower base exctatons. All the records normalzed to ther maxmum ground acceleraton as seres of (0.1g, 0.2g, 0.3g, 0.35g, 0.45g, 0.5g, 0.6g, 0.7g, 0.8g, 0.9g, 1g and 2g) and all of them would be baselne corrected. PGA has been selected as ntensty measure crtera on developng fraglty curve [Vamvatskos and Cornell 2002]. Selected records along wth ther characterstcs are descrbng on Table 3: Table 3. Earthquake Detals Earthquake PGA(g) Locaton Moment Magntude Year 3.3. Frequency Analyss Tabas 0.85 Iran Bam 0.81 Iran Naghan 0.73 Iran Elcentro 0.3 USA Kobe 0.5 Japan Cape Mendocno 0.8 USA Hollster 0.38 USA Frequency analyss has been performed to derve transtonal mode shapes and frequences. As a summery, t s obvous that ths structure categorzed as a low frequency structure. Frequences are llustrated n Table 4. Table 4. Frequency Analyss Result Mode Frequency(Hz) Perod(Sec) 1st nd rd Analyss Method Nonlnear dynamc approach has been performed for tower analyss. For ths purpose, acceleraton tme hstory records are mported to the software. Totally 84 analyss are performed. Maxmum Base shear of the tower s derved. Dampng rato s assumed 5%[Vamvatskos and Cornell 2002]. 4. DEFINITION OF LIMIT STATES The authors ddn t access any predefned structural lmt state of damage ndex for ths specal structure, therefore they performed a nonlnear statc pushover analyss wth a unform pattern loadng along the tower heght and the base shear related to the frst plastc hnge s selected as lmt state (see Fgure 4).

6 Fgure 4. Pushover Curve of tower 5. DEVELOPING FRAGILITY CURVES AND NUMERICAL RESULTS The dstrbuton of maxmum Base shears, estmated from the analyses, s shown n Fgure 5. The sold lne represents the mean value of the results. Lognormal dstrbuton was better ftted than normal dstrbuton to results n general. Fgure 5. Maxmum base shear of tower n each PGA levels The fraglty value at each PGA (F (PGA)) s estmated by changng the notaton of Equaton 3 and replacng the dstrbuton of damage ndex (f m (d)) by lognormal dstrbuton of ISD (f(sd) = φ[ ln(isd),β sd ] : F(sd) = P(D > d SD = sd) = 1 P(D d SD = sd) = 1 Φ (1/β.ln(ISD /ISD ˆ )) sd sd (5.1) Where, ISD s the mean ISD (result) threshold of damage states. Fraglty curve shown n the Fgure 6 are estmated by fttng a lognormal cumulatve dstrbuton functon: 1 sd F(sd) = P(D > d SD = sd) = Φ( ln( ) β SD (5.2) Where SD and β are mean and devaton of the functon respectvely [K Nasserasad et all 2009].

7 Fgure 6. Fraglty curve for structural performance of tower By revewng the results, ths condton could be predctable, because none of the results was on structural lmt state range. 6. CONCLUSIONS AND PROPOSALS FOR FUTURE WORK By studyng fraglty curves wtch presented on Fgure 6, the followng conclusons could be stated: 1. At the acceleraton more than 1.0g, the probablty of structural fraglty at the peredefned damage level has exceeded more than 10 percent. 2. The general concluson about the structural performance of ths system s that t has hgh relablty even at very hgh peak ground acceleratons. The authors propose future works conduct on some addtonal feld, for example: consderng the effect of tower's underlyng sol and sol structure nteracton (SSI), dervng better ntensty measure for fraglty curve such as S a, studyng the behavor of nonstructural equpment of power transmsson networks, dynamc nteracton of cable on structure. REFERENCES Thala Anagnos. (1999). Development of an Electrcal Substaton Equpment Performance Database for Evaluaton of Equpment Fragltes.Report Prepared for Pacfc Gas and Electrc and the Pacfc Earthquake Engneerng Center. C. Alln Cornell, Fatemeh Jalayer, Ronald O. Hamburger, Douglas A. Foutch.(2002). Probablstc Bass for 2000 SAC Federal Emergency Management Agency Steel Moment Frame Gudelnes. JOURNAL OF STRUCTURAL ENGINEERING Porter, K.A. (2003). An overvew of PEER s performance-based earthquake engneerng methodology. Proc. Nnth Internatonal Conference on Applcatons of Statstcs and Probablty n Cvl Engneerng (ICASP9). Volume: Jalayer F. (2003). Drect probablstc sesmc analyss: mplementng non-lnear dynamc assessments, PhD thess Cvl and Envronmental Engneerng Department, Unversty Stanford, Calforna. Cornell, C. A., Krawnkler. H. (2000). Progress and challenges n sesmc performance assessment. PEER Center News. 3:2. K. Nasserasad, M. Ghafory-Ashtany, S. Eshgh and M.R. Zolfaghar. (2009). DEVELOPING SEISMIC FRAGILITY FUNCTION OF STRUCTURES BY STOCHASTIC APPROACH. ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING). Volume: 10, NO. 2. Cornell, C. Alln, Jalayer, Fatemeh, Hamburger, Ronald O., and Foutch, Douglas A. (2002). The probablstc bass for the 2000 SAC/FEMA steel moment frame gudelnes, ASCE Journal of Structural Engneerng, Vol.: 128, No.4,

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